An RF cavity filter includes plural cavities formed therein to pass only a signal of frequency band in use, and is employed generally at a base station, etc. using comparative high power of a frequency signal.
The skirt characteristics and the insertion loss are important in view of the cavity filter. The skirt characteristics means a slope of a boundary band in a pass band characteristic curve and the insertion loss means loss at the input/output of the cavity filter.
The skirt characteristics is improved as the number of poles is increased, and the insertion loss is inversely proportional to the number of poles. In other words, the skirt characteristics has trade-off relation with the insertion loss, therefore the number of poles is determined by considering the skirt characteristics and insertion loss.
In general, the number of poles in cavity filter corresponds to the number of cavities, and the number of cavities is associated directly with size of the filter.
With the development of mobile communication, usage of the filter is being expanded, miniaturization and high-performance of the filter is required continually. A multi-mode filter is one of the filters developed according to the requirement.
The multi-mode filter uses multiple resonance modes in a single resonator unlike a single-mode filter. Therefore, the multi-mode filter is smaller in size compared to the single-mode filter, but has the advantage of high performance.
Meanwhile, conventional multi-mode filter uses port feeding. In other words, conventional multi-mode filter is fed by one of electric field coupling (E field coupling) and magnetic field coupling (H field coupling).
However, port feeding by electric field coupling without ground structure could not be applied to a cavity filter according to surge standard because the cavity filter according to surge standard must be grounded when port feeding is performed.
In addition, conventional multi-mode filter using port feeding by magnetic field coupling as shown in FIG. 1 could not obtain sufficient coupling amount to realize wide-band characteristic.